Specifically, a tent area inside of the main playing area at the 2007 World Series of Poker. The "tent" was used to hold final tables which were being simulcast on the internet and therefore had to be held in secret to prevent player collusion.
Sequestrium was originally thought to be a portmanteau of atrium and sequester but after outrage at the entire idea of preventing family, friends and fans from viewing final table action, many felt the entire idea was insane, so the combination of sequester and sanitarium was suggested.
Generally, any hiding or sequestering of events that should or could be open to the public.
The Pentagon is the largest sequestrium in the world.
The sequestrium at the 2007 World Series of Poker was a public relations nightmare.